1. Field Of The Invention
This invention relates to epoxy adhesive compositions. More particularly, the invention relates to epoxy adhesive compositions which are suitable for use in bonding rigid fiber-reinforced plastic composite materials to other materials of like or dissimilar composition, including metal materials which are coated with corrosion protection coatings.
The use of rigid fiber-reinforced plastic composite materials in the form of sheet molding compounds (commonly referred to as SMC) as an alternative for steel automotive body panels in an effort to reduce weight and corrosion susceptibility is increasing. Sheet molding compound can be described as reinforced resin compositions typically comprising unsaturated liquid polyester resin, ethylenically-unsaturated monomer, low profile thermoplastic resin, inert filler, magnesium oxide or other maturation aids and shaped or milled glass fiber reinforcement. The molding of the SMC under heat and pressure results in the formation of a rigid, self-supporting, thermoset, fiber-reinforced SMC part.
The rigid SMC parts are typically bonded to other rigid SMC parts, which may or may not originate from the same manufacturer, or to metal panels and structural members, in lieu of mechanical attachments. Reactive multi-component thermoset adhesives such as peroxide-cured unsaturated polyester, polyol polyurethane and epoxy adhesive compositions have typically been the adhesives most commonly employed in bonding these fiber reinforced plastic composite SMC materials. However, there are no adhesives whose performance is sufficiently stellar as to preclude the further development of structural adhesives for bonding reinforced plastic composite SMC materials.
Moreover, automotive adhesives must perform over a wide range of operating conditions, including temperatures on the order of 200.degree. C. which are encountered during paint bake cycles to below 0.degree. C. in cold weather climates, and environmental conditions such as exposure, oils, greases, humidity and high salinity. For example, one specification requires that SMC/metal adhesives withstand a torsional shear impact loading of 17 in. lb. per sq. in. at -29.8.degree. C. Standard epoxy adhesives cured with polyamidoamine curatives barely meet this minimum value, with the failure mode being adhesive to the metal surface. If the metal is coated with an organic corrosion inhibiting coating, the failure is typically between the corrosion inhibiting coating and the metal, rather than the desired failure within the SMC adherent. Another specification commonly required of automotive adhesives is that they be capable of bonding SMC parts without the need to further prepare or treat the SMC surface prior to bonding.
Attempts to improve the low temperature impact performance of SMC/metal assemblies by increasing the flexibility of the adhesive have resulted in impact values per se. However, the failure mode remained adhesive at the metal interface or between the corrosion-inhibiting coating and the metal, when the metal was so coated. In addition, the flexibilized epoxy compositions suffer from a decrease in cohesive strength at higher temperature. These failures have been alleviated in large part by curing the epoxy resin with a combination of amine curing agents that include polyamines, polyamides and amine-terminated acrylonitrile-butadiene rubbers. However, the cure rate and hence the rate of initial lap shear strength development of these adhesive compositions is less than desired.